A Multiscale Constitutive Model for Intragranular Ductile Damage during Sheet Metal Forming Processes

UK manufacturing industry is striving to develop precise methods to produce complex sheet metal components with improved accuracy and low process cost. For new products, design and development of the die is a time consuming and costly process; this process requires rigorous testing using the required sheet metal material to see if it will successfully form the sheet to the final product without reaching the forming limit (failure). To avoid cost and time involved during testing, computational predictive tools can play a significant role in reducing the number of trials during the design and development process. Therefore material constitutive models that can predict all ingredients of deformation in metals, such as elastic, plastic and damage are required.

This First Grant proposal addresses the problem of failure at microstructure level which is related to industrial forming found in many automotive and aerospace industries. This research project will aim to reduce the cost of producing sheet forming tooling by developing a computational framework which can be used to simulate the intragranular ductile damage in polycrystalline sheets due to void growth and coalescence with special emphasis on sheet metal forming. Damage models at macroscale are already available in the literature, however such models are purely phenomenological and do not include the physics of the microstructural deformation. The outcome of the proposed research will be a micromechanics based constitutive model that will incorporate the physical mechanisms (such as void growth and coalescence) during intragranular failure in polycrystalline materials. The proposed constitutive model will account for the inherent microvoid growth and coalescence inside individual grains, which is one of the major reasons of ductile failure in metal under high stress triaxiality during sheet metal forming. The research output will significantly advance the internationally-leading role of the UK in the sheet metal forming process simulation and failure prediction at sub-micron level.

The proposed research covers four different aspects from academic and industrial perspectives; these include understanding of underlying physical micromechanisms, development of a continuum based constitutive model using applied mechanics and mathematics principles, numerical implementation of these principles and finally simulation of the sheet metal forming process. The proposed research falls into three broad prioritized research themes of EPSRC: Physical sciences, Mathematical sciences, and Engineering. The proposed work will help in optimizing sheet metal forming and is therefore aligned with the EPSRC theme - Manufacturing the Future.

The proposed research evolved from detailed discussions at the Advanced Forming Research Centre (AFRC). These discussions took place during a series of arranged meetings and workshops with Tier 1 industrial partners, including Rolls Royce PLC, Boeing, Barnes Aerospace, and Timet. These industries are seeking a better understanding of sheet metal forming processes at sub-micron scale to optimize the process and to have a predictive tool to optimise the process with reduced numbers of experimental trials to save time, money and labour. Interest in the proposed research falls into the following categories:
- The developed constitutive model and computational framework will be applicable to different metal forming processes including sheet metal forming in UK industries. Designers, stress engineers, process engineers, process developers, and manufacturers will benefit from the framework to model and predict the deformation processes at submicron scale.
- The new method will provide an alternative powerful way of understanding and predicting microstructure evolution and failure during simple and complex uni-/bi-/tri-axial deformations.
- The theoretical and computational mechanics community will benefit from a broadening of the general theoretical results that may be applied to other deformation processes.

To apply and exploit the proposed research different mechanisms are already in place. These are all linked with AFRC's partnership within the HVM catapult collaborative programmes, such as Strategic Affordable Manufacturing in the UK through Leading Environmental Technologies (SAMULET), High Value Manufacturing Catapult Centre, and a newly formed Simulation Forum comprising of different catapult centres (AFRC, AMRC, CPI, MTC, NAMRC, NCC, WMG) all over the UK. Amir has been heavily involved as a technical resource in the above mentioned mechanisms on various projects and the proposed work (micromechanics based modelling) is one of the areas where the above centres do not focus. The proposed work will not only result in the in-depth understanding of microstructure evolution during sheet metal forming but will also provide a valuable predictive tool to model different types of forming processes at sub-micron scale.
Academic beneficiaries from the research will include those researching in the area of forming processes which involve finite (large) deformations - impact will be ensured through publication of research results in the appropriate journals and conferences.
Lastly, it is anticipated that there will be an economic (and potentially social) impact of the research to the industry and general public, as the computational framework developed during this work will reduce the number of experimental trials. This will save the amount of time and money spent on producing a part which ultimately will reduce the overall cost of the product.